Rochester 2 Jet Fuel Percolation

Throttle Body Venting

I ran across some information about some of the 2 jets that deals with the percolation of gas after shutting off the vehicle. We hear about this problem a lot these days because gas now has a lower boiling point.

Percolation means that the gas is boiling resulting in a very strong fuel mixture. This can make a hard to start situation.

I’m not sure if this was done on every 2 Jet, but it wouldn’t be hard to figure out if you have this feature just by looking at the throttle body (float bowl side).

The purpose of throttle body venting is to give quicker hot engine starting after the engine had been shut down for a short period.

During extreme hot engine operation the fuel in the carburetor tends to boil and vaporize due to engine heat. I said extreme, but gas now has a lower boiling point and it doesn’t take a lot of heat for percolation to happen. Some of the fuel vapor tends to reach the carburetor bores and condense on the throttle valves and seep into the engine manifold. By venting the area just above the throttle valves, hot engine starting time can be reduced to a minimum, on applications where the carburetor is exposed to extreme engine heat.

 throttle vent

There are 2 methods used in venting the throttle bore area.

1. A special throttle body to bowl gasket is used. See figure A. This gasket has cut-out areas which vent fuel vapors from the carburetor bores just above the throttle valves.

2. The other type of venting is accomplished by drilled holes through the throttle body casting just above the throttle valves. See figure B. They serve the same purpose as the vented gasket.

The location of the vent holes are such that they will not disrupt engine idle or off idle operation. They are located above the throttle valves on the side opposite the mixture screws, in an area where the transfer from idle to main metering will not be affected.

Now don’t go out and cut holes to create the vent holes if you don’t now have them. That isn’t going to work. You will most likely create a vacuum leak.

6 Responses

  1. don george

    have 69 chevelle with 2bbl now have rebuilt carb and new gas tank flushed lines new fuel pump all fuel system new gas comes out top and throtle bores runs fine several mins then quits or runs very bad then you shut off engine see fuel come out of bores to then put gas on manifold again all new fuel system checked float and vit valve filter all ok so what happened why this problem also put on new gas cap vented don george thanks

    • I didn’t see where you tested the fuel pump pressure. That would be the 1st thing. Too much pressure would force too much fuel. You can get several ideas on what the problem may be here:

      • don george

        have new holly fuel pump and shows correct pressure of little under 5lb at 2000 rpms so i dont think i need a reg as i believe pressure is ok don

      • don george

        ive ordered another carb as i cant seem to understand all fuel system is new i now wonder if i bought a insulator which goes under carb if that would have fixed it but now ill wait for the new carb to come said 10days ok will let you know if new carb cures or same don

      • don george

        the shop i have car frame off restore put on there manifold and a q type carb works perfect much power but cant use on mine can only use a 2bbl as orginal for shows but car must be dependable its not at this time so ill wait for replacement carb will tell you if fixed don

  2. You always have good information on carburetors, period.

Leave a Reply